There are several different techniques for producing a lightguide fiber for use in communications. One such technique comprises directing a constantly moving stream of reactants and oxygen through a glass substrate tube having a generally circular cross-section. The oxygen stream carries silicon tetrachloride and dopants to produce the desired index of refraction in the finished lightguide fiber. The substrate glass is heated to a reaction temperature within a moving hot zone that traverses the length of the tube, and the consequent reaction produces doped silicon dioxide fused into a continuous layer on the inner wall of the tube. The resulting tube is referred to as a preform tube.
A torch assembly for heating a glass substrate tube to facilitate deposition is described in U.S. Pat. No. 4,231,777 which issued on Nov. 4, 1980, in the names of B. Lynch and F. P. Partus. See also U.S. Pat. No. 4,401,267 which issued on Aug. 30, 1983 in the name of C. D. Spainhour. Initially, one end of the tube is supported in the headstock of a lathe and the other end is welded to an exhaust tube that is supported in the tailstock. Combustible gases are directed through a housing and nozzles of the torch assembly and toward the tube as it is turned rotatably about its longitudinal axis and as the torch assembly is moved therealong on a carriage to produce a moving hot zone. A temperature profile is produced across the hot zone which moves along on the surface of the tube, with a peak value sufficient to accomplish the desired reaction and deposition. See F. P. Partus and M. A. Saifi "Lightguide Preform Manufacture" beginning at page 39 of the Winter 1980 issue of the Western Electric Engineer.
During a deposition mode, the torch carriage moves slowly from the headstock of the lathe where dopants are moved into the glass tube to the tailstock where gases are exhausted. At the end of each pass from headstock to tailstock, the torch carriage is returned rapidly to the headstock for the beginning of another cycle. The ends of the nozzles adjacent to the tube are cooled to eliminate substantially degradation by oxidation or reduction, for example, of the material forming the housing and nozzles. In one embodiment of this technique, an rf plasma is established in the tube to enhance certain processes in reaction and deposition. See for example, U.S. Pat. No. 4,331,462 which issued on May 25, 1982 in the names of J. W. Fleming, Jr., J. B. MacChesney and P. B. O'Connor. See also U.S. Pat. No. 4,262,035.
Subsequent to the deposition mode, a collapse mode is used to cause the preform tube to become a solid rod-like member which is called a preform. It is this preform from which lightguide fiber is drawn. See D. H. Smithgall and D. L. Myers "Drawing Lightguide Fiber" beginning at page 49 of the hereinbefore identified Winter 1980 issue of the Western Electric Engineer. The process of collapsing a preform tube may consume as much time as three and one-half hours. In order to collapse the preform tube, the torch assembly is moved in a number of passes from tailstock to headstock. The temperature of the moving hot zone which is higher during the collapse mode than during the deposition mode softens the tube wall and allows surface tension to cause the tube to collapse into a rod.
What is needed and what is not provided by the prior art are methods and apparatus for collapsing, in a relatively short period of time, a preform tube into a preform from which lightguide fiber is drawn. Inasmuch as the collapse mode is performed on the same lathe which is used during deposition, reduced time for collapse will enhance the production of preform tubes. Seemingly, the prior art has not addressed this problem.